How Neurons Generate Signals The Neuron at Rest

Stepping on a Thumbtack  Reflexive withdrawal of the foot – a simple behavior controlled by a circuit with direct connections between: Sensory neurons (responding to environment) Motor neurons (controlling muscles) Interneurons (to inhibit opposing muscles) Projection neurons (carrying sensation to the brain)  Messages are carried via action potentials.

Biochemical Reactions  Ion – an electrically charged atom.  Polar covalent bonds – a molecule held together by sharing electrons (H 2 O).  Ionic bond – a molecule held together by the attraction of atoms with opposite charges (NaCl – table salt).  Cation – ion with a net positive charge.  Anion – ion with a net negative charge.

The Neuron at Rest  Neurons have potassium (K +) inside and sodium (Na + ) outside in the extracellular fluid.  Ion channels in the cell wall (membrane) are selectively permeable to potassium, sodium or calcium.  Ion pumps maintain the cell’s inner environment.

How Ions Cross the Membrane  Diffusion – an ionic concentration gradient exists  Differences in electrical membrane potential and equilibrium potential Ionic driving force  Ion pumps Sodium/potassium, calcium

Electricity  Opposite charges attract, like charges repel.  Current (I) – movement of electrical charge, measured in amps.  Potential (voltage, V) – force exerted on a particle, difference between terminals, measured in volts.  Conductance (g) – relative ability of a charge to migrate, measured in Siemens.

Resting Potential  Membrane potential is voltage across the neuronal membrane.  Resting potential is the point at which all of the forces acting upon ions are in balance (equilibrium). Diffusional and electrical forces are equal. V m = -65 mV

Two Important Equations  Nernst equation – calculates the equilibrium potential for a single ion, given knowledge of its concentration inside & outside the neuron. Varies with body temperature.  Goldman equation – calculates the resting potential of a neuron, given knowledge of its permeability to various ions. Varies with ion concentrations.

Regulation of Potassium  The neuron at rest is mostly permeable to potassium (K+) so the resting potential is close to E K.  Increasing potassium outside the cell leads to depolarization (an increase in the resting potential making it less negative).  To prevent this, potassium is tightly regulated by the body via glia & blood-brain barrier.